The present invention relates to gas-generating compositions, more specifically to gas-generating compositions used in a gas-generating apparatus for expanding air bag of vehicle or in a passenger-protecting apparatus, such as a pretensioner device that takes up a seat belt.
Recently, safer non-azide gas-generating compositions are researched and developed, in place of gas-generating compositions that comprise mainly of inorganic azide compounds, such as sodium azide having hazardous properties. Specifically, gas-generating compositions having a high gasification rate can achieve a decrease in size and weight of the gas-generating apparatus since the loading amount of gas-generating compositions to the gas-generating apparatus is reduced. Recently, gas-generating compositions having gasification rate exceeding 90% are required.
If the influence on the passengers is considered, the combustion residue of the gas-generating compositions is preferably substantially neutral. Gas-generating compositions are required to produce little or no carbon monoxide.
Ammonium nitrate has drawn attention as an oxidant for non-azide gas-generating composition. Ammonium nitrate has the advantages that it does not produce a hazardous gas or a combustion residue, is low in toxicity, and it is relatively safe. However, ammonium nitrate lowers the combustion rate of gas-generating composition when used as an oxidant. Accordingly, in order to improve combustion rate of gas-generating composition, metal oxyacid salt, such as potassium nitrate, sodium nitrate and strontium nitrate, is added. Further, the volume of ammonium nitrate changes along with phase transition. Potassium oxyacid salt, such as potassium nitrate, is added to prevent degradation of gas-generating composition form.
Oxyacid salts of transition metals are generally inappropriate because they have properties difficult to handle (deliquescence, toxicity, low thermal stability) Therefore, oxyacid salts of alkali metals or alkali earth metals are used. Oxyacid salts of alkali metals and alkali earth metals have a problem in that they release strong basic combustion residue outside the gas-generating apparatus, after combustion.
PCT-based Japanese Patent Application Laid-Open No. Hei 9-503194 discloses a conventional gas-generating composition containing phase-stabilized ammonium nitrate as the main component, triamino guanidine nitrate, and organic polymer binder. Japanese Patent Application Laid-Open No-2000-169276 discloses a conventional gas-generating composition comprising phase-stabilized ammonium nitrate, nitroguanidine, and oxygen-containing potassium salt. However, a large amount of the basic combustion residue (potassium hydroxide and potassium carbonate) is released after the combustion in conventional gas-generating compositions.